The present invention relates to gain control in a radio receiver path. In particular, gain control is provided in a signal path with a sigma-delta analog-to-digital converter.
Gain control is provided in receivers, such as receivers for mobile cellular telephones. Automatic gain control promotes the most efficient use of the whole dynamic range of an analog-to-digital converter in the receive path. The automatic gain control allows the use of the receiver in various propagation conditions (as fading, etc), maximizing the signal-to-noise ratio at the input of the Analog-to-digital converter and avoiding clipping along the receiver line-up. Gain normally is provided by analog baseband gain stages in the receiver back-end, and in the receiver front-end it is provided by RF Low-noise amplifiers (LNA) and mixers.
A variable gain amplifier after the mixer provides gain control in the analog baseband stages. An example of a common post-mixer amplifier (PMA) is a simple inverting amplifier with the gain set by the ratio of the feedback resistor (Rf) to the input resistor (Ri). The great amount of gain variation (dynamic range) to be provided by this single amplifier would require a great amount of current. Greater dynamic range implies a large variation of the feedback resistor, and, by consequence, the operational amplifier would need a higher current in order to drive the minimum resistor value correspondent to the minimum gain with a minimum distortion. The low distortion request arises because in order for the line-up to tolerate the third-order (or second-order) intermodulation components that fall in-band a high IP3 or IP2 is required. However, high quality amplifiers for dealing with distortion as well as an increasing dynamic range are more expensive in terms of current. In mobile telephones, current and expected standards require a large dynamic range. For example, the 3 G specification requires close to 90 dB of signal dynamic range. Of course this huge dynamic range requirement would be divided between the RF and low-frequency analog blocks. However, since the RF blocks cannot have as large gain variations as the baseband blocks due to instability of RF blocks as the gain changes, most of the gain variation normally is assigned to the baseband analog blocks. For example, a PMA has a voltage gain variation range between −12 dB to 10 dB, implementing a 22 dB AGC dynamic range. An operational-amplifier based PMA that provides a gain from −12 dB to 10 dB needs to vary the feedback resistor from 502 ohms to 6.3 k. The operational-amplifier would need a large current to drive this low resistance of 502 ohms when the gain is −12 dB. As a result, substantial current is needed in the line-up. Requiring extra current may reduce the battery life on a mobile telephone. While the dynamic range may be divided between the LNA and the variable gain amplifier of the baseband stage, increased requirements for the LNA or baseband amplifiers may still be undesirable.